it depends...If you mean for breakfast, there is no charge. They will even give your child his/her own serving at no extra charge. If you mean for lunch or dinner, you will be charged what you order for your little one -- unless s/he really does eat off your plate...but you can order a children's meal, if appropriate.
I've been to lots of restaurants in WDW with DD#1 age 18 months, and then again with DD#1 aged 2 1/2 yrs and DD#2 8 months...If it was a buffet or pre-plated meal(unlimited/family style), no charge for under 3. If it was an order each item off a menu meal, then we would be charged for whatever we ordered.
Hope that helps!